Skip to main content

Using a Java stream (Java 8+) to convert List collection into primitive data type arrays



You need to convert an ArrayList<Integer> to int, similar case for others like Long > long.

Prior to Java 8, you would have to try something like
int[] myarray = ll.toArray(new Integer[size]);
But this will fail to compile! So it forces you to use Integer[] instead of int[]. However, you may have client code that expect int[] and not Integer[]. So the only other way would be to iterate over the list and convert to array the traditional way by adding element by element. Fine, but not elegant.


I discovered that the stream based method above works even for primitive data types like int.

So converting an array list of integers to an array is quite straightforward with a mapToInt() intermediate Intstream (or LongStream for long) as follows
ArrayList<Integer> ll = new ArrayList<Integer>();     
int[] myarray = ll.stream().mapToInt(c -> c).toArray();
The c-> c is just a lamda that says return the value c for each value in the stream, unmodified. Because this is an IntStream stream, it has a method that returns int[] and this works for this case.





Labels:

Comments

Post a Comment

Popular posts from this blog

Authenticating Spring Boot based application against secure LDAP/AD server

Authenticating against an Active Directory setup is quite common in organizations using Spring Boot / Spring Security can be a pain if you don't know exactly the requirements. I needed to add auth in my web app and secure some but not all endpoints of the application. My story was, I needed Spring security to authenticate against my company LDAP server which uses Active Directory I started by using the standard LDAP guide such as this which are all over the Internet, https://spring.io/guides/gs/authenticating-ldap/ and was able to setup the basic framework However, only test level LDAP auth was working for me, when I tried to auth against the company LDAP secure server, I had to resolve a few issues After 1 week and working with several devs at the company, I finally found why it was not working and the fix was easy Since I spent a week or so resolving this, I wanted to write this up in case someone finds this useful. Here is what I did (it was easy until the fourth ...
Post a Comment
Read more

Unit testing code that uses environment variables and system properties with fakes

I did not exactly learn this today, but I am sharing it as I have found it extremely useful when unit testing code that depends on environment or system property settings. While I am using Java as an example, the general concepts apply any where. Problem : You have a piece of code you are unit testing that uses settings from env variables or system properties passed to the VM (System.getProperty), but you don't want the tests to be affected by the 'real' environment or system properties in the VM. So, your unit tests should not get different results or fail when the real environment changes. Solution : There are several. But the most straightforward is to use a mocking library to mock out the environment or fake it out, whatever your prefer. You can create a fake using a library like EasyMock, PowerMock etc. This I won't discuss in this post, since there are numerous articles for that. Or you can write a simple class that acts as a proxy, using the proxy pattern...
Post a Comment
Read more

Using custom conditional logic to enable/disable Spring components

If you have a Spring component and you don't want it to load, you can use Spring's predefined conditionals as much as possible. For example, @Component   @ConditionalOnNotWebApplication   public class SchedulerEntryPoint implements ApplicationRunner { ...  } This will not load your component when running in non web application mode. Such as you may want to start the application but without any of the web framework using SpringApplicationBuilder. But sometimes you want to use custom conditions. It's pretty easy to do so, just use something like this @Component @Conditional (SchedulerCheck. class ) public class SchedulerEntryPoint implements ApplicationRunner { public static class SchedulerCheck implements Condition { @Override public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) { return System. getProperty ( "scheduler" ) != ...
Post a Comment
Read more